The pedal inputs were changed from an on/off style, to a trigger style so the dd75 kick pedal can be velocity sensitive. You can attach a yamaha kp-65 trigger with an actual kick pedal to it.
That’s actually how the bass pad works on the Yamaha DD models. You have to step on it while on the floor. It actually doesn’t work by pressing it while holding it.
Good question tmk. There are no USB ports on the old Yamaha DD50s. It has a quarter inch jack output that I plug into a Roland amplifier. I'll take a look and see if it has anything else, but that is the output I have always used.